1. Field of the Invention
The invention relates to the characterization of samples which are present in many hundreds to tens of thousands on a sample support plate precisely positioned in a regular array, by measurements such as mass spectrum acquisitions with ionization by matrix-assisted laser desorption (MALDI) with a narrowly focused laser beam in a mass spectrometer, for example.
2. Description of the Related Art
Combinatorial chemistry methods with thousands of synthesized samples are currently experiencing a revival, especially for investigating reactions of biopolymer samples with antibodies, with synthesizing or degrading enzymes, or with oxidizing or reducing chemicals. For example, photochemical methods can be used to assemble 100,000 different peptides, which cover the amino acid sequences of all human proteins, strongly adhering on sample supports the size of microscopy specimen slides. These peptides can then, for example, be subjected together to reactions with a specially selected phosphorylase in order to see at which locations in the whole sequence of the human proteome this enzyme exerts its effect. Alternatively, it is possible to bring such peptide arrays into contact with serum or plasma, for example, in order to determine the peptides to which blood constituents bind specifically. The findings thus obtained could be used to screen for auto-antibodies, for example.
Experiments of this type can provide researchers in biochemistry, and in pharmaceutics in particular, with a lot of valuable information. But these experiments require an analytical method that can, at the least, unequivocally indicate which samples on the sample support have reactively changed. Even more advantageous is an analytical method which can indicate the type and position of the reactive change within the sample molecules.
Microscopy can only be used to a limited extent, for example if reactions are accompanied by changes in color or fluorescence. Surface plasmon resonance (SPR) methods, especially imaging SPR, can be used, but have limitations in respect of the sample size. It is also possible to use completely different methods, such as micro-Raman, infrared or UV spectrometry, to determine special types of reaction.
The most advantageous method for high-throughput characterization of samples is provided by mass spectrometry, however. J. H. Lee et al. have already shown that they were able to correctly analyze 5,000 samples on a specimen slide with a coated area of 23 mm by 54 mm (High-Throughput Small Molecule Identification Using MALDI-TOF and a Nanolayered Substrate; Analytical Chemistry, 2011, pubs.acs.org/ac). The authors developed a method which they used to produce sample areas with a diameter of 300 micrometers (with 500-micrometer grid spacing in a square array) each individually coated with a matrix. The position of the array in the mass spectrometer was determined by means of the integrated camera and, as a safeguard, with the aid of 36 equally sized sample spots containing reference substances within the array.
This method reaches its limits if the density of the samples is to be increased significantly. Particularly when the samples are synthesized on the sample support in monoatomic layers, it is no longer possible to recognize them by visual means. In addition, at least for ionization by matrix-assisted laser desorption, matrix substance must be added to the samples afterwards, and homogeneous overcoating can hardly be avoided. The video camera installed in the mass spectrometers can therefore no longer be used to determine the position of the sample array; the positions of the samples must be determined by other means. This applies not only to mass-spectrometric measurement, but also to other types of measurement method.
In view of the foregoing, there is a need to provide instruments and methods with which the position and orientation of a sample array, which cannot be recognized by visual means, on a sample support whose position in an analytical instrument is not known with sufficient accuracy, can be precisely determined to within a few micrometers in order that every sample area can be utilized as completely as possible for characterization of the samples, and particularly for mass-spectrometric characterization with a small-area scan.